1. Field of the Invention
This invention relates to the improvement in material for use as an electrode of a distributor which conveys a high voltage from the ignition coil to each cylinder of an internal combustion engine. More particularly, it relates to the distributor which is capable of suppressing radio noise caused by spark discharge which is made between the rotating electrode and each of the fixed electrodes.
2. Prior Art
A distributor is a device for conveying a high voltage generated in the ignition coil to each sparking plug in synchronization with the engine processes through spark discharge made in a narrow gap between the rotating electrode and the fixed electrodes.
The current of the spark discharge is composed of capacitive discharge current and induced discharge current.
The capacitive discharg current is a high frequency current which flows instantaneously (for several fractional seconds) with a sharp rise at the instant when the electric charge accumulated in the capacity between the rotating electrode and the fixed electrodes, or accumulated in the floating capacity between the high voltage cable which connects the ignition coil to the rotating electrode and the earth and between the electrodes nearest to the discharge gap and the earth is discharged when dielectric breakdown is caused in the discharge gap (the narrow gap). As a result, the electromagnetic wave caused by the high frequency current is radiated outside, e.g. through the high voltage cable, etc. which serves as an antenna and thereby may cause radio noise.
On the other hand, the induced discharge current is of low frequency (tens to hundreds mA) which flows continuously at the instant when the capacitive discharge ends. Ignition energy supplied to the sparking plugs amounts almost in proportion to the product of the discharge duration time of the induced discharge current and the induced discharge current.
Therefore, the radio noise can be suppressed without losing ignition energy by reducing only the capacitive discharge current.
The capacitive discharge current is reduced by the following conventional methods which have, however, the disadvantages described subsequently:
METHOD (1): employing a resistor
In this method, a resistor is embedded in the rotating electrode or each of the fixed electrodes, or a high resistive layer is formed on the surface of the rotating electrode or each of the fixed electrodes. This method has disadvantages that radio noise suppression effect is small at the high frequency zone up above 300 MHz and ignition energy loss caused by the resistor is large.
METHOD (2): widening the discharge gap
In this method, discharge gap between the rotating electrode and each of the fixed electrodes is widened up to 1.5-6.4 mm.
This method has disadvantages, such as an extremely large loss in ignition energy due to the widening of discharge gap and the corrosion of the electrodes caused by corrosive gas which affects metals, such as nitrogen oxide (NOx) which is generated by higher discharge voltage between the rotating electrode and each of the fixed electrodes, although it has large radio noise suppression effect.
METHOD (3): employing a new material for electrodes
In this method, silicon carbide (Japanese unexamined published patent application No. 119730/1977), ferrite (Japanese unexamined published patent application No.38447/1979), conductive ceramic (Japanese unexamined published patent application No. 75969/1981) or sintered body of a mixture of ceramic and ferrite (U.S. patent application Ser. No. 639,204/1984) is used for at least either the rotating electrode or the fixed electrodes to replace the conventional material of brass.
Noise suppression effect provided by the electrodes made of one of the materials is generally adequate but at the high frequency zone up above 300 MHz it is not sufficient. Also this method has disadvantages, such as large ignition energy loss at too high resistivity, or overcurrent flowing at too low resistivity, and local heating due to the low coefficient of heat transfer which is sufficient enough to affect the durability.